WO 2008/135502 describes a security device, which is used as a defense against copying and counterfeiting of articles such as bank notes, passports and authenticity documents.
In the above application a circuit is provided comprising an antenna, a rectifier and an EC-display, wherein the antenna is arranged to receive electromagnetic radiation from an external source and to convert it into electric energy. The rectifier is arranged to receive electric energy from said antenna and convert said energy to a rectified current. The electrochromic display is arranged to receive rectified current from said rectifier, and to alter its electrochromic state in response to said rectified current in order to indicate the authenticity of e.g. a security document. According to a preferred example, electromagnetic radiation from a mobile phone is used to active the security device.
There is a constant demand on lowering the manufacturing cost of security devices, especially when these are to be applied to low cost articles, such as bank notes. Thus, there is a demand for components which are printable and which may operate at high frequencies, such as at 0.4 GHz and above.
Previously the problem of rectifying high frequencies has been solved by conventional semiconductor devices like for example surface mounted device (SMT). However such devices are not printable. Today, the reported printable diodes (p-n diodes) have been shown to work up to about 10 MHz.
Lilja K. E. et al., ‘Gravure printed organic rectifying diodes operating at high frequencies’, Org. Electronics (2009), doi:10.1016/j.orgel.2009.04.008 describes diodes operating at common RFID frequencies, such as 10 MHz. The diodes comprise conducting layers comprising silver and copper, respectively, with an organic polymer layer sandwiched in between. However, this diode is not suitable for operation at frequencies of 0.4 GHz and above.
Thus, there is a need for a printable diode working at frequencies of 0.4 GHz and above. There is also a need for a manufacturing process facilitating large scale production of such diodes with cost effective methods.